1. Field of the Invention
Production of oil by any of the known reservoir-producing techniques, as for example water drive, gas cap drive, dissolved gas drive, gravity drainage, etc., leaves significant quantities of oil in the reservoir. The most efficient of these displacement mechanisms, whether it is a primary (naturally occurring) or secondary (rejuvenating) operation, is displacement of oil by water. However, even this technique leaves from one-tenth to one-third of the total reservoir pore space filled with oil.
When water attains a certain local saturation during a water drive or flooding operation, the continuous oil filaments break into disjointed segments which are entrapped and held immobile by capillary forces. Surface-active agents or surfactants have been employed to reduce the magnitude of the capillary forces in an attempt to prevent entrapment of the oil, or to free the oil after it has been trapped. In certain instances, where the oil is held at least partly by its adhesion to the rock surfaces, surface-active agents also may act as a detergent.
Surfactants may be added to the flood water or preferably, the surfactants may be produced in situ. Producing surface-active agents in situ has advantages over the simple introduction of surface-active agents to the floodwater, because when produced in situ the surface-active agents are concentrated in the interface between the oil and water where they are most needed, and because of this, they do not contact the rock surfaces with resultant adsorption by the rock surfaces. Also, variations in the interfacial tension which result from differences in concentration of the surface-active agents at different places in the interface cause localized stirring and turbulence which tend to cause the trapped oil to emulsify itself spontaneously. Further, the interfacial tension during the transient phase in which a surface-active agent is coming to equilibrium between the oil and water phases may be reduced below its equilibrium value by a factor of 100. This is believed to result from the fact that a much larger concentration of surface-active agent is maintained in the interface by in situ formation than can be obtained at equilibrium when the total concentration of surface-active agent is in the water phase.
One way in which the surface-active material can be formed in situ is by alkaline flooding. In alkaline flooding alkali present in the flooding material reacts with petroleum acids in the oil to produce surface-active agents at the oil-water interface. In cosurfactant-enhanced alkaline flooding a preformed surfactant is added to the flooding material. Thus the "primary surfactant" is formed by reaction in the formation and the added preformed surfactant is the "cosurfactant."
Some oil reservoirs contain large quantities of calcium sulfate (anhydrite) dispersed throughout the reservoir rock. Calcium sulfate is a slightly soluble salt which provides a constant source of calcium ions. These ions react with the alkali, e.g. NaOH in the cosurfactant-enhanced alkaline flooding material. The equation for this reaction is: ##STR1## The effect is continuous consumption of the alkali with the result that it is impossible to maintain the preferred high concentration of hydroxide ion in the flooding material. Since high hydroxide concentration provides high pH, it is thus impossible to maintain a high pH in the flooding material.
In the method of this invention, the hydroxide concentration (high pH) in the flooding material is maintained by the addition to the flooding material of an alkali metal sulfate or ammonium sulfate.
2. Prior Art
U.S. Pat. No. 3,584,686 issued to Fulford relates to a method of minimizing deposition of calcium sulfate in a water flooding process by adjusting the total mineral salt content to a high enough concentration such that the solubility of calcium sulfate does not decrease with a decrease in the water flood pressure. Flooding media salt concentration may consist of brine alone or in combination with a controlled amount of other salts including sodium sulfate in order to adjust the salt content of the fluid drive.
U.S. Pat. No. 3,091,291 issued to Little et al. discloses a method of treating an underground formation with ammonium bicarbonate in order to prevent calcium sulfate formation. The exact amount of bicarbonate needed to be maintained in the injection fluid is measured by converting all the sulfates of calcium into ammonium sulfate when calcium carbonate is precipitated.
U.S. Pat. No. 3,175,610 issued to Osoba discloses removing undesired calcium ions from a flood water by first flooding the formation with a solution that is higher in sodium chloride concentration than the water in the formation. This solution displaces calcium from the surfaces of the rock. Injection of floodwater containing a lower sodium chloride concentration than the preceding solution then follows the injection of the higher sodium chloride concentration solution, and the floodwater is stripped of its calcium by the rock surfaces.